Introduction: Osteosynthesis is the reduction and fixation of a bone fracture with an implantable device. The device is usually made of metal, either titanium or stainless steel, and may include plates, pins, rods, screws, and staples. It is a surgical procedure with an open or percutaneous approach to the fractured bone. The procedure aims to bring the fractured bone segments together and immobilize the fracture site while healing takes place. The use of osteosynthetic devices has a long history and a reasonable record of success.
Bone Fractures: A bone fracture is a medical condition in which there is a break in the continuity of the bone. A fracture may result from either a high impact force or a low impact force, frequently as a result of a medical condition that weakens the bone, such as osteoporosis. Bone fractures can be complete or incomplete, simple or comminuted, closed or open. Comminuted fractures comprise more than two bone fragments, and open fractures are associated with an open skin wound. While many fractures are treated non-operatively, a large number of fractures require surgical intervention. If a fracture that requires operative treatment is not treated, nature tries to stabilize the mobile fragments by pain-induced contraction of the surrounding muscles, which may lead to bone shortening. The end result of this process frequently is the lack of proper bone alignment and impaired function.
For lower extremity fractures, stability for weight bearing is the main goal. In the upper extremity, restoration of functional hand and arm motion is most important. For diaphyseal fractures, proper alignment of the fracture fragments is typically all that is needed for adequate function and prompt healing of the fracture, whereas intraarticular fractures require precise anatomic reduction with articular congruency being very important.
Fracture Fixation: The basic goal of fracture fixation is to stabilize the fractured bone, to enable fast healing of the injured bone, and to return mobility and function of the injured extremity. This is accomplished either by using conservative treatment methods (non-surgical) or by using mechanical fixation devices (surgical). Mechanical fixation devices may either be external to the body (i.e., ring fixation), or internal to the body (i.e., rods, pins, and plates).
There are two types of fixation, flexible (biologic) and rigid. With flexible fixation, the fracture fragments displace in relation to each other when load is applied across the fracture site. Fracture fixation is considered flexible if it allows appreciable interfragmentary movement under functional load. Fracture healing under flexible fixation typically occurs by means of callus formation.
Rigid fixation provides immobilization and allows fractures to heal by the process of intramembranous ossification. Rigid fracture fixation is desirable for fractures that involve articulating surfaces. Bridging of the fracture with a stiff splint reduces mobility of the fracture fragments and allows minimal displacement under functional load. Articular fractures require exact anatomic reduction and stable fixation to avoid development of abundant callus. This is important because unevenness of the joint surface and presence of callus formation at the articular surface lead to patient discomfort and often development of early and progressive osteoarthritis.
The only technique that effectively abolishes motion at the fracture site is interfragmentary compression which uses a plate and screws to completely eliminate motion at the fracture site, and allows for direct healing without formation of visible callus. However, in some cases, over time, the stainless steel and titanium fixation devices do not maintain compression across the fracture fragments. The reduction of compression has been observed to be as much as 32% over a two week period. Additionally, as the necrotic surfaces of the fracture are resorbed, a non-load bearing gap develops between the fragments, thereby decreasing compression and increasing the risk of interfragmentary motion and scar tissue formation.
Loss of compression is contrary to the objectives of fracture fixation in general. Excessive interfragmentary motion results in the formation of fibrous, unmineralized scar tissue (resulting in a non-union or pseudoarthrosis) versus the regeneration of bone. The unmineralized scar tissue does not support load and may lead to loss of skeletal function. In addition to compression across the fracture site, a sufficient blood supply must be maintained to support skeletal metabolism, bone regeneration, and remodeling of the fracture site.
Conservative Fracture Treatment: Conservative treatment utilizes non-surgical methods to restore the alignment and subsequent stabilization of the affected bones. Conservative treatment is achieved by traction or by external splinting. Traction devices are temporarily applied along the long axis of the bone, to align the bone fragments and provide some stability. See FIG. 1, which shows a simple traction device.
External Fixation: External fixators provide fracture fixation based on the principle of splinting. They are the only systems that allow the surgeon to control the flexibility of the fixation system. External fixators are the standard in treating open fractures that present with substantial soft-tissue injuries and require additional vascular procedures, fasciotomy, soft-tissue flaps, or multiple debridements, to avoid additional damage to an already compromised limb. Additionally, external fixation may be indicated for polytrauma, fractures in children to avoid pin fixation through the growth plate, temporary joint bridging before later open reduction internal fixation (ORIF), and arthrodesis of the ankle, elbow, or knee. In these latter cases, external fixators are especially indicated in acute or chronic infections, in limb-lengthening procedures, and occasionally after corrective osteotomies.
External fixators are made of pins or wires (Schanz screws, Steinman pins, Kirschner wires) that are placed percutaneously into the bone above and below the fracture site. These pins or wires are connected by various clamps to external fixation rods (stainless steel or carbon fiber rods). There are two basic types of external fixators: standard pin fixator ring and hybrid fixator. Standard uniplanar external fixators consist of percutaneously placed pins that are connected to an external rod. Proper pin or screw placement is very important. These pins or screws should penetrate the near cortex and medullary canal and engage the far cortex without penetrating the muscle compartment. Joint penetration by any of these pins must be avoided. Standard uniplanar fixators can be used for almost every long bone fracture except those involving the proximal femur or humerus. They are commonly used for the stabilization of complex distal radius fractures. The pins are placed in the distal radius and second metacarpal shaft. This technique uses the surrounding soft tissues or ligamentotaxis to provide indirect stabilization of the fracture. See FIG. 2, which shows an external fixation device.
Internal Fixation: Since the late 1950s, open reduction internal fixation (ORIF) has been used to restore bone anatomy and enable early mobilization and to overcome the limitations encountered when fractures are treated with skeletal traction or cast immobilization. The main goal of internal fixation is the achievement of prompt and, if possible, full function of the injured limb, with rapid rehabilitation of the patient. The majority of internal fixation implants are currently made of stainless steel. Occasionally, less strong but biologically superior and more elastic titanium implants are favored. Numerous devices are available for internal fixation. These devices can be roughly divided into a few major categories: wires, pins and screws, plates, and intramedullary nails or rods. Staples and clamps are also used occasionally for osteotomy or fracture fixation.
Pins: Fixation pins can be smooth or threaded and are made in a large number of sizes. Among the most commonly used are Kirschner (K) wires and Steinman pins. These devices are used for temporary fixation of the fracture fragments during fracture reduction, and as guides for the accurate placement of larger cannulated screws. The percutaneously placed Kirschner wires commonly protrude through the skin for ease of later removal. Occasionally, the pins are used for definitive fracture treatment (skeletal stabilization) and should be watched for migration. The Steinman pin is also occasionally used for wrist arthrodesis. See FIG. 3, which shows Steinman pins.
Wires: Wires are used alone or more commonly in combination with other orthopedic fixation devices. They are of various diameters and may be braided. Wires are frequently used to re-attach osteotomized bone fragments (i.e., greater trochanter or olecranon). In combination with pins or screws, the wires are used to create a tension band, which uses muscular forces to create compression at the fracture site. Wires are used to suture bone and soft tissue, and while wires may break, if there is no loss of bone fragment position, breakage of wires is usually of little significance. Circumferential cerclage wires are commonly used in conjunction with intramedullary fixation to stabilize long bone fragments. One of the potential complications with cerclage wires is interruption of the periosteal blood supply with subsequent osteonecrosis or fracture nonunion. See FIG. 4, which shows examples of wires used in orthopedic fixation.
Screws: Screws are of different sizes and can be self-tapping (i.e., have cutting ends) or non-self-tapping (i.e., require a prepared tapped hole). Non-self-tapping screws are easier to insert and remove, but they are not the best choice for fixing fractures in regions with a thin cortex. Some screws have a “standard” point and others a “trocar” point. Screws are commonly used in combination with plates and nails or rods. The use of different types and designs of screws depends on the surgeon's preference. Screws may be cannulated (i.e., have a hollow central shaft) so that they can be accurately implanted using a guide wire.
There are two basic types of fixation screws, cortical and cancellous. Cortical screws are often fully threaded and usually have a smaller thread diameter and pitch. They are designed to be used in the diaphysis. Cancellous screws are intended to cross long segments of cancellous bone. They typically have deeper threads, larger thread diameter, and a greater pitch than cortical screws, and they are usually partially threaded, with threads only on their ends. Occasionally, cancellous screws can be fully threaded. See FIG. 5, which shows common orthopedic screws.
Loosening of well-placed screws is induced by micro motion at the interface between the thread and bone. From a radiologic standpoint, it is important to observe and report possible complications including screw breakage, loosening, or changes in position.
Plates: Plates can be used for both rigid and flexible fracture fixation. The majority of these plates are made of stainless steel or titanium. See FIG. 6, which shows an exemplary orthopedic bone plate.
The terminology that is commonly used with fracture plating is “compression plating” and “neutralization plating.” Compression plating applies compression to the fracture ends. In cases of severely comminuted fractures, bone loss, or other situations that prevent compression, the plate is screwed into place to hold the fracture fragments together during healing. Frequently, not all the screw holes in the plate are filled. When diaphyseal fractures in the long bones are treated with a plate, a minimum of six cortices should preferably be engaged at each fracture site, except for the femur, which requires eight. Plates are most commonly used for fixation of long bones, but they also are used in the spine and for arthrodesis of the wrist.
The dynamic compression plate (DCP) has holes designed for axial compression, which is achieved by means of eccentric screw insertion. The DCP functions in different modes: compression, neutralization, tension band, or as a buttress. It is available in different sizes to accommodate fracture fixation in bones of different sizes. The screw holes in the DCP are oval and are best described as a portion of an inclined and angled cylinder. The plate can be used with different types of screws. From the radiologic standpoint, important considerations are the location of the plate, whether the plate symmetrically spans the fracture, and the degree of fracture reduction. The plate should not impinge on joint motion, and the plate and the screws should not violate the articular surface. A major complication with plating is the potential compromising of cortical blood supply because of the large contact area between the plate and the underlying cortex. See FIG. 7, which shows a dynamic compression plate.
Intramedullary Nails or Rods: Most intramedullary nailing is done closed with minimal soft-tissue exposure, either in an antegrade or retrograde fashion depending on the fracture site. Both antegrade and retrograde nailing are used for femoral and humeral shaft fractures, and for tibial shaft fractures antegrade nailing is used. The entrance site for an antegrade femoral nail is created in the piriformis fossa; for the retrograde femoral nail, in the intercondylar region; and for the antegrade tibial nail, anteriorly just below the joint line. The nails are introduced over a guide wire, frequently after reaming with flexible reamers to enlarge the intramedullary canal. See FIG. 8, which shows fixation using an intramedullary rod.
Many different designs of intramedullary nails and rods are available. Femoral nails are bowed anteriorly to accommodate the contour of the bone. A majority of nails are cannulated to allow their placement over a guide wire. Intramedullary nails provide excellent stability against bending forces, but they do not control rotation and compressive forces. For the control of rotational forces, proximal and distal interlocking screws are placed (usually in a lateral to medial fashion) through the nail or rod holes in the proximal and distal femur. Interlocking screws increase fixation stability and therefore lead to an increased use of nailing in fracture fixation. Interlocking screws also prevent collapse or shortening of the fracture. If the nail is locked both proximally and distally, it is “statically locked” because all planes of motion are controlled or static. The nail is “dynamically locked” if it is locked at one end only, which allows compression at the fracture site. See FIG. 9, which shows an intramedullary nail with screws.
Flexible intramedullary rods are of smaller diameter and greater flexibility to accommodate different long bone anatomy. Flexible rods are inserted through the metaphysis. They are frequently used for fixation of long bone diaphyseal fractures in skeletally immature patients to avoid placement through the growth plate and subsequent premature closure of the growth plate. Multiple flexible rods, which diverge in the metaphyseal regions, are placed through multiple insertion sites. These rods provide some axial and rotational stability. For small-diameter bones, sometimes a single flexible rod is used. The major disadvantage associated with flexible rods is the frequent need for additional external stabilization such as a plaster cast.
Short intramedullary rods with transversely and obliquely oriented interlocking screws are used for fractures in the diametaphyseal region that extend into the adjacent joint. The weakest points of these intramedullary rods are the distal interlocking screws. In children with osteogenesis imperfecta, two-part telescoping rods are used to allow lengthening of the rod as the child grows.
Potential complications with intramedullary rods include change in bone length, distraction of the fracture site, hardware fracture, hardware loosening, and infection. Intramedullary rods and interlocking screws should not violate the joint surface. The contraindications for intramedullary nailing are local or systemic infection, femoral fractures in patients with multiple injuries, pulmonary trauma (for which temporary stabilization with an external fixation device is recommended) and metaphyseal fractures (for which fixation with interlocking screws may be insufficient to control malalignment). Additionally, because the bone is reamed to insert the rod, there is temporary damage to the internal cortical blood supply which is associated with increased infection rates. There is also an increased rate of pulmonary complications including pulmonary embolism with reaming.